Ayyyyyy I am starting to play lux more, but I have a problem with mid. Naturally, I go to the role I'm most comfortable with: support. I can land most of my skill shots, but I accidentally take my adc's kills, don't get the concept about my w, and I don't really know what skills to level up while playing support lux. Can someone help me please?
It's the same as Mid lux. Don't worry about taking kills, you need them to be impactful in the game, but don't go out of your way to KS either. Because of the nature of Lux's kit, you're going to take a lot of kills if you're playing correctly. It's not like the gold is wasted, and your ADC can farm while the enemy is dead. Use your E's utility to scout the bot brush and zone. Rank your E first, then add points in Q if you're snowballing or W if you're behind to focus on supporting.
If you snowball build like an APC, Rabadon's Morello's Frost Queen's etc.
If you fall behind, buy more supportive items like Watcher's, Redemption, and Locket.
Ideally I'd pick her into short range enemy mage supports like Brand/Annie/Malz, and I'd avoid her against tanky all-ins like Blitz/Leona/Thresh.
Also /mute everyone but your ADC unless they're toxic, then mute them too. You need to play by your own instincts, not like a 0 AP Janna like your teammates might try to bully you into playing.
Start E. Lower cooldown, slightly more damage. Easier to hit, and you can weave a basic attack in with it for fast thunderlord procs. Also can be used to grant vision in unwarded places. Be careful not to spam it, as the mana costs will drain you fairly quickly.
Items: Slellthief's, upgrade to either FCQ or the Eyestone. Ionian boots. If you're snowballing, I suggest Luden's. Otherwise I like to grab GLP-800 for the additonal slow and health/mana boost. From there is mostly situational items. Abyssal, Morello (not super efficient to build late, but sometimes you need Grevious Wounds), Lich Bane. Generally, you want to max CDR, and get an item that will give you a decent mana pool so you can afford your spells.
Always take TLD.
Max R > E > Q > W.
Passive makes for really sufficient trading pre-level 4 with TLD. She has a long AA range, so use that in lane to poke. Use Q Defensively more so then offensively as it's your only real peel. Don't max W Until you have maxed E and Q first, the scaling is garbage and so are the stats. Use E to Zone as opposed to poke. Q + R, and then Leaving E underneath the enemy can guarantee your ADC almost any kill without you stealing it. Popping your E pre-maturely will normally result in you taking your ADC's kill, which you do not want to do.
Marksman is the highest scaling role in the game, as much help as you might be with 14 kills and 600AP, it's not going to change the fact Lux is garbage at taking towers, which in the end is what you need to do to win. Consider building some AP if your midlaner is an AD champ like Zed or Talon.
Build Ardent Censer. Lux's W allows her to apply the Life Drain and AS buff to everyone on your team with one Basic ability, she's the only champion who can do this, take advantage of it.
Majority of the time you are better off building Support items then AP. You will be low on gold as a support, there is no point in sinking 3800 Gold into a rabadons when you can buy 2 Support items for the same price. Frost Queens, Ardent Censer + Runes and Masteries will give you sufficient AP.
And as ALWAYS Build Sight Stone, Redemption and Locket of the Iron Solari. Both items are extremely broken right now.
Typical Build Consists of 
and your own boots of choice out of 
I'll normally run Magic Pen boots, but Mobi's if behind or Swifties if they have lots of slows.
Try not to mute your teammates unless necessary. They might have useful information. If they're salty you might have no choice.
Have fun.
